Deep Water Nutrient Supply for an Offshore Ulva sp. Cultivation Project in the Eastern Mediterranean Sea: Experimental Simulation and Modeling

Offshore cultivation of marine macroalgae is a potential sustainable resource for fuel, food, and chemicals. Offshore, the high productivity of macroalgae cultivation depends on external nitrogen supply. The current work examines the idea of supplying nitrogen for Ulva sp. cultivation in the oligotrophic Eastern Mediterranean Sea (EMS) by artificial upwelling of nutrient-rich deep seawater (DSW). Growth rates, protein, and starch contents of Ulva sp. were measured for time varying fertilizations with nitrate concentrations corresponding to nutrient concentrations of DSW at increasing depths of the EMS. A maximal relative growth rate of 7.4% was measured for fertilizing ten times per week with 5.8 μM, which corresponds to the artificial upwelling from the depth of 700 m at EMS. Protein and starch contents ranged between 1–6 and 8–15% of dry weight. Finally, yields and energetic costs of DSW pumping were modeled for an example case of 10-ha offshore farm. The model predicts a high productivity but low energetic efficiency, which can be improved by coupling the biomass production with offshore power sources such as ocean thermal energy conversion.